1. Field of the Invention
The present invention relates to apparatus and methods for impedance matching in microwave and millimeter wave circuits. More particularly, this invention pertains to an auxiliary flange for adjusting the impedance of a device of the type that includes a port of the rectangular waveguide type, within a microwave or millimeter wave circuit.
2. Description of the Prior Art
The coupling of microwave and millimeter wave energy within microwave and millimeter wave circuits is highly dependent upon the design of the physical circuit elements and transmission lines. Quite often, electromagnetic incompatibilities in the physical dimensions of interconnected devices create undesired reflections of energy that limit power transmission. On the other hand, it is often desirable and beneficial to design some reflectivity elements into microwave and millimeter wave circuitry to reduce power transmission to less-than-maximum levels. For example, some microwave tube applications cannot run at maximum power without harm to the surrounding environment.
The important regulation of power transfer within millimeter and microwave circuitry is often implicit in the design of circuit elements. Designs commonly incorporate "stubs" and the like for affecting input and output impedances. By the deliberate creation of impedance mismatches between devices, a predetermined degree of power transfer may be obtained. Conversely, through careful matching of device impedances, it is possible to obtain near-total power transfer.
The design of microwave and millimeter wave circuits that possess desired impedance characteristics is well-understood by those skilled in the art and is discussed, for example, in N. Marcuvitz, "Waveguide Handbook", Radiation Laboratory Series, v. 10 (MacGraw-Hill 1951). As mentioned above, a recognized element for adjusting the impedance of a microwave circuit is the stub which, when inserted into the path of propagation of energy, will introduce a disturbance that mimics a change in either the capacitance or the inductance of the associated circuit. The character of the effect will depend upon the physical relationship of the disturbance to the rest of the circuit at the frequency of interest.
While careful circuit design can employ stubs for creating desired impedance values, for a number of reasons it is often quite difficult to adjust impedance levels after the fact. For example, optimum design will vary with changes in operating frequency. This may require one to discard an otherwise useful device designed for operation at one frequency rather than put it to use in a different application. Manufacturing tolerances (or design errors) frequently result in the production of devices that fall outside optimum performance levels.
The ability to correct impedance levels and to thereby avoid discarding potentially-useful devices is highly prized. The correction of impedance mismatches is, unfortunately, not a simple task at present. The post-manufacture correction of impedance levels is very difficult in some cases. Such difficulty is quite often related to the peculiarities of structure and operation of particular types of microwave and millimeter wave devices. These factors may prohibit the use of physically-intrusive methods of adjustment. For example, the interior of a microwave tube must be evacuated and support a vacuum. Ceramic "pill box" windows seal the ports of the tube, each comprising a section of loaded rectangular waveguide. To adjust the tube's impedance after manufacture, one must break the vacuum to obtain interior access, a costly and time-consuming step that severely limits the ability to perform trial-and-error adjustment methods. Accordingly, the designer is limited to analytical solutions, restricting his ability to "fine tune" devices to compensate for factors that cannot be adequately modeled mathematically, thereby forcing reliance upon such recognized sources such as the "Waveguide Handbook" ibid, and well-known Smith Chart procedures. In addition to depriving the designer of an option, analytical methods require relatively high-skilled labor that renders the process of adjusting impedance quite costly.